It is well known to measure fluid flow or relative velocities of a fluid with respect to a sensor, using devices which sense pressure and/or variations of pressure of the flowing fluid. Normally, those devices are connected to gauges or other control apparatus or signal handling circuits which are or may be calibrated so as to give a reading of the parameter being sensed. That parameter is most usually velocity, but given a constant cross section of a duct system which may be a duct, pipe, shaft, conduit or tube, volumetric flow may be determined. Moreover, as will be discussed hereafter, fluid flow sensors--especially of the type disclosed by this invention--may be used for such purposes as the determination of static pressure within a duct system; and, of course, one of the principal purposes for constant monitoring of fluid flow is to sense changes in that flow so that flow control apparatus such as fans or pumps, dampers or baffles, etc., may be actuated.
Still further, it is possible in given circumstances, such as the measurement of air flowing in heating and/or air conditioning ducts within a building, to install temperature sensing thermocouples within fluid flow sensors, for more complete control and/or monitoring of the fluid flow circumstances being sensed.
Very often, fluid flow sensors may be variations of the simple pitot tube, of the sort which may also be used for determination of air speed--that is, relative flow of a body to air, or air to the body--of aircraft and the like. Pitot-type flow meters are well known in the industry, as are other kinds of sensors such as hot wire sensors--where electrical performance characteristics of a hot wire installed within a duct may be electrically determined, by sensing variations of voltage and/or current and/or resistance as the hot wire characteristics are affected by the fluid flow. Or other devices having perforated plates or specially formed grids may be inserted within a duct, where pressure and/or volumetric measurements are made by sensing and measuring flow parameters from one side to the other of the device, and/or against the device.
Especially when measuring flow characteristics within a duct system, where the dimensions of the duct, tube, pipe or shaft may be relatively large--in excess of several inches, and up to several feet in width, height or diameter--it is important to sense fluid flow at many points across that large dimension, so as to have a more representative signal or integration of signals from which meaningful data concerning fluid flow characteristics may be derived. Thus, except in very small ducts or pipes, or in installations where careful calibration over a lengthy period of time have taken place so that measurement of fluid flow characteristics at a single point may be indicative of the fluid flow characteristics within the entire duct system, it is not usually advisable to rely upon single point sensing devices. Therefore, such devices as pitot tubes and hot wire sensors are not normally acceptable for close tolerance and meaningful data derivation of fluid flow characteristics.
The more usual condition, therefore, is to use a device which provides signals that are more closely and realistically derivative of or indicative of an average or mean fluid flow parameter being sensed, across one or a number of diagonals within a duct system at the point where the sensor or sensors is or are may be installed. However, in nearly all such cases, a calibration chart is required; and depending on the nature of the installation, consistent and repeatable data may or may not be easily obtained. For example, it is usually accepted that devices that are installed by the manufacturer into a duct system, including both the sensor and the gauge, are more consistently repeatable as to the ability to derive meaningful flow data from them. Other circumstances, where an installer may place the sensor and gauge, or where the manufacturer has placed the sensor but the installer carries a gauge, result in less predictably consistent accurate flow measurements from place to place and/or from time to time.
It is therefore one of the principal purposes of the present invention to provide a fluid flow sensor which not only will be applicable for high and low velocity fluid flow circumstances, but which will give meaningful and consistently repeatable readings for any given fluid flow conditions. A specific design of the sensing element of the fluid flow sensor of the present invention is such that there should be negligible flow pressure drop across the device, within the duct system in which it is installed, and negligible regenerative noise values that will affect fluid flow measurements.
Of course, fluid flow sensors according to the present invention are such that variations in velocity pressure across the duct system where the sensor is installed are averaged, so that signals derived from the sensor are indicative of the mean fluid flow and are therefore more representative of the actual fluid flow conditions.
The present inventor has discovered, quite unexpectedly, that if a fluid flow sensor is installed in a duct system, and has two independent pressure chambers where the first pressure chamber extends across a major dimension of the duct in a manner so as to be substantially perpendicular to the direction of flow of the fluid within the duct, and where the axes representing the mean pressure in each of the two independent chambers are parallel one to the other, with the first chamber being upstream of the second chamber, and with the first chamber being in fluid communication with the fluid flow through a plurality of small passages at the upstream side of the sensor while the second chamber is in fluid communication with the fluid flow other than at the upstream side of the sensor, then there will be a multiplying effect of the differential pressures between the dynamic or velocity pressure which is the resultant of the fluid flow, and the static pressure which is the resultant of the presence of the fluid within the duct, so that variations in the dynamic or velocity pressure may be sensitively determined because those variations may be measured as the product of the consistent multiplier and the multiplicand which is the difference between the total pressure sensed in the upstream chamber and the static pressure sensed in the downstream chamber. Of course, the consistent multiplier is a constant which is a function of the geometry of the sensor, but remains constant over a wide variation of differential pressures and within a wide variation of flow characteristics within the duct system.
Still further, the present inventor has found that when the fluid flow sensor is generally cruciform in profile, and has upwardly and downwardly directed wings which are placed between the independent pressure chambers so as to be between the fluid communication passages of those pressure chambers relative to the fluid flow, a multiplier which may be in the range of from 1.5 to 5.0 (most often in the range of about 3.5) is obtained. This permits sensitive determination of pressure flow differences, in either high velocity or low velocity fluid flow conditions, whereby the sensitivity of the fluid flow measurement may be predicted and repeatably consistent.
The prior art includes LAMBERT, U.S. Pat. No. 3,751,982, issued Aug. 14, 1973. That patent shows a device having two chambers, one of which faces upstream and one of which faces either across stream or downstream, with a plurality of openings into each chamber. LAMBERT recognizes that the difference between the total pressure and the static pressure for low volumetric rates of air flow may not be sufficient to operate a means for measuring differential fluid pressure; and he therefore states that for effective operation of that means the difference must be increased by a factor for which compensation may then be made in the means for measuring differential fluid pressure. He provides the increased difference linearly by providing sets of downstream or rearward facing openings, but he goes to great lengths to teach that the openings in the rearmost chamber may either face crosswise of the fluid flow, or rearwardly, but not both. Neither does LAMBERT provide any indication as to the magnitude of the apparent multiplier effect, which in any event is stated to be applicable only at low volumetric rates of air flow.
HARBAUGH et al, in U.S. Pat. No. 4,154,100 issued May 15, 1979, teach a pitot-type flow meter which has a downstream facing port, where considerable effort is given to the design of upstream and downstream facing surfaces so as to affect boundary layer flow characteristics; all so as to overcome the inherent difficulties of pitot-tube sensing.
RENKEN et al, in U.S. Pat. No. 4,344,330, issued Aug. 17, 1982, provide tubular members that are formed in a loop which is transverse to the fluid flow, with one of them having orifices which faces upstream and the other having orifices which face downstream. By configuring the tubes in the manner discussed, with a plurality of orifices, RENKEN et al are confident that they measure and react to average air flow conditions where the sensor is installed. RENKEN et al not only do not recognize any multiplying effect, they teach away from it, as to the derivation of signals and the way they are handled by their average fluid flow sensor.
VICTOR, in U.S. Pat. No. 4,425,807, issued Jan. 17, 1984, teaches a two chamber device having upstream facing openings to a first chamber, and pairs of openings to a second chamber which are oriented at an angle of about 110.degree. to the upstream facing openings. By having the rear holes with that particular angle, it is said that there is a substantially consistent flow co-efficient which is independent of the conduit's Reynold's number, over a relatively wide range of useable fluid velocities. VICTOR is particularly concerned with the desire to remain relatively independent of Reynold's number for a given fluid flow situation, and is very specific as to the orientation of pairs of rear openings with respect to forward openings in the range of 105.degree. to 115.degree., preferably 110.degree..
One other prior art patent is that issued to ENGELKE, U.S. Pat. No. 4,453,419, issued June 12, 1984. That patent relates to a commercially available product which has pairs of sensor tubes, which in certain embodiments may be mounted to an extruded holding device having blades which extend in both directions from the tubes. These blades shape the flow of air as it passes over the downstream sensing tubes, so as to permit pressure at the downstream side of the tubes to be more accurately detected. There is no discussion, however, of any multiplier affect, and if such exists it is ignored as being something which is not easily accommodated because the principal purpose of the ENGELKE invention is to create a resultant differential pressure signal which is highly representative of the actual volumetric flow rate, and is therefore directly indicative of the actual gauge pressure differences of the flow parameters.